The present invention relates to a process for fabricating nuclear reactor fuel pellets having large grain sizes from highly active UO.sub.2 powder. In particular, the present invention concerns a method for controlling the sintered density of UO.sub.2 pellets to a predetermined range.
When UO.sub.2 pellets are used as the fuel in nuclear reactors, it is important that the fuel density be high within a reasonable limit so that a more compact reactor core can be designed, and that the thermal conductivity of the pellets is sufficiently high.
However, when the sintered density of the pellets is too high, swelling of the pellets during irradiation becomes too great, thereby damaging a tube in the reactor. Accordingly, UO.sub.2 pellets commonly used in light-water reactors are usually designed so that the sintered density is in the range of from 94 to 97% TD (theoretical density).
One recent technical innovation is to prolong the useful life of the reactor fuel. This is called the plan for "high burnup", and it is now being studied seriously. In order to execute this plan, it is imperative to restrain the fission gas (FP gas) in the pellets as much as possible.
It is well known that producing large crystal grain sizes is effective in confining FP gas in the pellets. However, the conventional technology only produced grain sizes of at most about 10 to 20 .mu.m.
In light of the above, the applicants have proposed a process for fabricating UO.sub.2 pellets with large-grain size crystals in JP-A-62-297215, JP-A-63-45127, U.S. patent application Ser. Nos. 139447, 296802 and 296808. These processes have a common effect of producing in that they make crystalline grains of large size by controlling the conditions of ammonium diuranate (ADU) formation.
With the processes described in the applications and patents, it is possible to control the crystalline grain size, however at the time these previous patents were filed, the applicants did not consider to control the sintered density of the pellets. In other words, when pellets are fabricated pellets having grain sizes larger than 20 .mu.m by the process described above, the sintered density of the resulting pellets are as high as 98 to 99% TD.
In order to reduce the sintered density of the sintered body, if required, it has been common to add a pore-former agent to the raw material powder, which cause the formation of pores when the agent sublimates during sintering. The applicants believe that the method is applicable to the fabrication process for pellets composed of large-grain size crystals. Although a pore-former agent of this kind is effective in reducing the sintered density, the agent is likely to have adverse effects on UO.sub.2 grain growth. In other words, the formation of crystalline grains with large grain size is disrupted. Accordingly, it is desired to develop a method for controlling both the sintered density of the pellets and the crystalline grain sizes.